1. Field of the Invention
The present invention relates to methods for purifying carbon monoxide and, more particularly, relates to a method for purifying carbon monoxide by removing metal carbonyls in high-purity carbon monoxide, which is used in an etching process in the semiconductor industry.
2. Description of Related Art
Because of high reactivity with metals, carbon monoxide produces various metal carbonyls on contact with various metals, and also on contact with metal constituting a cylinder charged with carbon monoxide or piping through which carbon monoxide flows; for example, contact with stainless steel produces metal carbonyls such as nickel carbonyl. Therefore, when using high-purity carbon monoxide in the semiconductor industry, a means of removing metal carbonyl from carbon monoxide must be provided at a point of use so as to reliably remove metal carbonyls which may be produced in the piping.
As a means of removing metal carbonyls in carbon monoxide, various methods have been proposed. Among these methods, a method of removing metal carbonyl by using a metal oxide having strong oxidizability such as copper oxide is known (see, for example, Patent Document 1).
Patent Document 1: Published Japanese Patent Translation No. 2001-521813 of the PCT Application (pages 2 to 3)
However, when conventionally used oxides, that is, metal oxides having strong oxidizability, such as copper oxide, manganese dioxide, and chromium oxide are used to remove metal carbonyls in carbon monoxide, the carbon monoxide itself tends to be oxidized, producing a large amount of carbon dioxide. When carbon monoxide containing a large amount of carbon dioxide is used in the etching process, there is a possibility that abnormal charge of the plasma and abnormal processing in the etching will occur.
According to a conventional treatment for removal of metal carbonyl, it has been believed that carbon dioxide as a reaction by-product may be removed by an adsorbent in a later stage, although particular attention was paid only to reactions such as oxidative decomposition of metal carbonyl. A large amount of carbon dioxide produced drastically increases the amount of the adsorbent in the later stage and requires replacement operations and regenerating operations, thus adversely affecting costs in the semiconductor industry.